Examples of related bulk material filling apparatuses have been disclosed in Japanese Patent Application Laid-Open No. (JP-A) 113362/79 (first related art) and Japanese Patent Application Laid-Open No. (JP-A) 52001/83 (second related art).
According to the first related art, first the volume of bulk material is measured with a massive filling measure and its weight thereof is measured with a measuring device. Then, the bulk material is poured continuously in a small quantity through a small hole and when the poured amount reaches an objective filling value, the small hole is closed such that the filling operation is completed.
According to the second related art, the weight of a package product in which its filling of bulk material is completed is measured with a weight checker and the tendency in the supply amount thereof from a cup measuring device is controlled. At the same time, a bulk material surface detecting device is provided on the hopper and according to signals output from this detecting device, the amount of bulk material flowing into the hopper is adjusted.
However, the related arts have the following problems.
(1) In the first related art, the small throw-in of the bulk material is conducted using the small holes, thus if the characteristics of powder change, the fluidity thereof is affected. Thus, if the fluidity thereof is deteriorated, the powder falling time is prolonged thereby reducing the filling accuracy.
(2) In the second related art, a result of measured weight of the package product after the filling is completed is fed back to the next filling operation. The flow rate cannot be compensated at real time. Thus, in this second related art as well, if the fluidity of bulk material changes, a high precision filling cannot be conducted.
(3) In both the first and second related arts, when the throw-in is terminated, the amount of bulk material (called falling amount Wh) existing in the air (falling height) from just below the outlet of a storage portion closed by the opening/closing device up to a filling object is considered. It is necessary to control the opening/closing device at a timing in which the filling weight W of bulk material filled in the filling object reaches W=W.sub.0 -Wh (W.sub.0 : objective amount). However, the falling height of bulk material varies depending on the size of a container which is the filling object. Thus, it is necessary to change the closing timing of the opening/closing device for every container size, so that a time required for changing a product type increases.